Most spectroscopy systems fall into one of two categories. They can be tunable source systems that generate a tunable optical signal that is scanned over a scan band. A detector is then used to detect the tunable optical signal after interaction with the sample. The time response of the detector corresponds to the spectral response of the sample. Such systems are typically referred to as pre-dispersive. Alternatively, a tunable detector system can be used. In this case, a broadband signal is used to illuminate the sample. Then, a bandpass filter is tuned over the scan band such that a detector time response is used to resolve the sample's spectrum. Such systems are typically referred to as post-dispersive.
Among tunable source and tunable detector systems, tunable source systems have some advantages. They can have a better response for the same optical power transmitted to the sample. That is, tunable detector systems must illuminate the sample with a broadband power signal that covers the entire scan band. Sometimes, this can result in excessive sample heating. Also high power is generated at the optical source, most of it being used, making the system inefficient. In contrast, at any given instant, tunable source systems only generate and illuminate the sample with a very narrow band power within the scan band.
Further, tunable source systems have advantages associated with detection efficiency. Relatively large detectors can be used to capture a larger fraction of the light that may have been scattered by the sample, since there is no need to capture light and then collimate the light for transmission through a tunable filter or to a grating and a detector array.
A number of general configurations are used for tunable source spectroscopy systems. The lasers have advantages in that very intense tunable optical signals can be generated. A different configuration uses the combination of a broadband source and a tunable passband filter, which generates the narrowband signal that illuminates the sample.
Historically, most tunable lasers were based on solid state or liquid dye gain media. While often powerful, these systems also have high power consumptions. Tunable semiconductor laser systems have the advantage of relying on small, efficient, and robust semiconductor sources. One configuration uses semiconductor optical amplifiers (SOAs) and microelectromechanical system (MEMS) Fabry-Perot tunable filters, as described in U.S. Pat. No. 6,339,603, by Flanders, et al., which is incorporated herein by this reference in its entirety.
In commercial examples of the broadband source/tunable filter source configuration, the tunable filter is an acousto-optic tunable filter (AOTF) and the broadband signal is generated by a diode array or tungsten-halogen bulb, for example. More recently, some of the present inventors have proposed a tunable source that combines edge-emitting, superluminescent light emitting diodes (SLEDs) and MEMS Fabry-Perot tunable filters to generate the tunable optical signal. See U.S. patent application Ser. No. 10/688,690, filed on Oct. 17, 2003, by Atia, et al., which is incorporated herein by this reference in its entirety. The MEMS device is highly stable and can handle high optical powers and can further be much smaller and more energy-efficient than typically large and expensive AOTFs. Moreover, the SLEDS can generate very intense broadband optical signals over large bandwidths, having a much greater spectral brightness than tungsten-halogen sources, for example.